Hello Matt, Thank you very much for your reply.
Here "S" is a symmetric matrix, so I have to use the CG. But I can not figure out how to use the Conjugate Gradient technique to find the solution. In the "MatShell" I can wrap my "MatVec" routine which calculates the "S*x" vector. But how will I be able to do the comparison between the known "b" vector and the calculated "S*x" using a CG (and come up with a new trial solution "x" for the next iteration). What is the function that I should call for this purpose? Thanking you. Pallab -----Original Message----- From: [email protected] on behalf of Matthew Knepley Sent: Thu 5/14/2009 5:03 PM To: PETSc users list Subject: Re: Conjugate Gradient technique On Thu, May 14, 2009 at 3:58 PM, Barai, Pallab <baraip at ornl.gov> wrote: > Hello, > > I am using PETSc to solve a set of linear equations "Sx=b". > > Here "b" is known and "x" is the trial solution. > > The complete (assembled) form of S is not known. That is why I am not able > to use something like "KSPSolve". > > But given a trial solution "x", I can calculate "S*x" using a "MatVec" > routine. > > Is it possible to use the Conjugate Gradient (CG) technique to find a > solution in this case? In place of "S", I can give "S*x" as the input. You can use CG if S is symmetric. If not, try GMRES. > > It will be great if someone can show me some direction. If this thing has > already been discussed before, the link to that thread will be sufficient. You can wrap your function in a MatShell: http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/Mat/MatCreateShell.html Matt > > Thanking you. > > Pallab Barai > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener
